The present invention relates to ink jet printers. Some aspects of the invention have particular application to continuous jet ink jet printers. A continuous jet ink jet printer is one in which, during the printing of a pattern or character, drops of ink are provided continuously and the printer is arranged so that drops which are not desired to create printed dots do not strike the surface on which a character or pattern is being printed.
Ink jet printers are well known, and are shown, for example, in U.S. Pat Nos. 3,298,030, 3,373,437 and 3,569,275. Further prior art, illustrating aspects of ink jet printers an providing background to aspects of the present invention, is shown in "Ink jet Printing" M. R. Keeling, Phys. Technol., Vol. 12 pp 196, published in Great Britain by the Institute of Physics, and in U.S. Pat. Nos. 3,681,778, 3,562,761, 3,465,351, 3,736,593, 3,683,396, 4,032,928, 3,600,955, 3,787,882, 4,417,256, 4,368,474, 4,638,325, 4,367,476, 4,631,549, 4,628,329, 3,631,511, 3,827,057, 3,875,574 and 4,384,295. All of the above-mentioned prior art documents are incorporated herein by reference.
In practice, ink jet printers do not always provide perfect print quality. Additionally, most ink jet printers require the operator to perform adjustments which are not always easy to carry out correctly. These problems are related, in that poor print quality is sometime caused by failure of the operator to carry out adjustments correctly, or sometimes even to carry out adjustments at all.
Reasons for poor print quality in prior art ink jet printers include incorrect amplitude of a modulation signal provided to a transducer for controlling the break-up of the ink jet into droplets, failure to adjust the operating parameters of the control system of each individual printer to match the particular characteristics of the individual print head being used, damage to or misalignment of parts of the print head which have to be moved to perform adjustment, cleaning or other operations, failure to maintain the correct ink viscosity and pressure, failure to perform printer start-up and shut-down routines necessary for optimum performance, failure to compensate the charging signal provided to an ink droplet charging electrode for individual variations in the performance of charging circuits, ink jet to charge electrode coupling and the effect of nearby ink droplets on each other, and the failure to maintain the correct ink jet velocity.
Ink jet printers may also be inconvenient to operate. In addition to requiring operator adjustments as referred to above, prior art ink jet printers may require operator intervention to initiate special routines when there are printing difficulties, such as a routine to clear a blockage from an ink jet nozzle. The versatility of an ink jet printer is greatly enhanced if a range of print heads are available providing different ink droplet sizes and speeds, but it is normally possible to change the print head on an ink jet printer only with great difficulty if at all. Ink jet printers frequently fail to operate correctly due to simple faults correctable by the operator, and possibly caused by incorrect operator adjustments, but such faults may cause the printer to be out of operation for considerable periods owing to the time taken for service personnel to arrive in order to diagnose the nature of the fault and the particular corrective action needed.
Prior art ink jet printers are frequently also complicated and expensive devices. Where it is desired to provide substantially identical printers having different print head nozzle sizes, so as to provide different ink drop sizes and production rates, it has in the past been necessary to provide completely different ink jet forming and modulating devices, as each device tends to be specific to a particular nozzle size and frequency of ink jet modulation.